H represents enthalpy which is the heat content stored in a chemical system.
ΔH of formation – enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. ΔH of formation is -ve
ΔH of lattice enthalpy – enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions. ΔH of lattice enthalpy is -ve.
ΔH of atomisation – enthalpy change when one mole of gaseous atoms forms from its element in standard state. ΔH of atomisation is +ve.
ΔH 1st ionisation energy – enthalpy change when one electron is removed from each gaseous atom in one mole to form one mole of gaseous 1+ ions. ΔH is +ve.
ΔH 2nd ionisation energy – enthalpy change when one electron is removed from each gaseous 1+ ion in one mole to form one mole of gaseous 2+ ions. ΔH is +ve.
ΔH 1st electron affinity – enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions. ΔH is -ve.
ΔH 2nd electron affinity – enthalpy change when one electron is added to each 1- ion in one mole to form one mole of gaseous 2- ions. ΔH is +ve because of repulsion between the negative ion and electron.
ΔH of solution – enthalpy change when one mole of a compound is dissolved in water under standard conditions. Can be endothermic or exothermic.
ΔH of hydration – enthalpy change when one mole of aqueous ions are formed from their gaseous ions under standard conditions.
Hess’ law states that if a reaction can take place by more than one route, and the initial and final conditions are the same, the total enthalpy change of each route is the same.
Lattice enthalpies cannot be calculated directly, because it is important to form one mole of an ionic lattice from gaseous ions. We can use a Born – Haber cycle to calculate lattice enthalpy indirectly.
Typically, an upward arrow represents endothermic reactions, while a downward arrow represents exothermic reaction.
Factors affecting Lattice Enthalpy
Ionic radius – as the ionic radius increases, the lattice enthalpy becomes less exothermic as the attraction becomes weaker.
Ionic charge – as the ionic charge increases, the lattice enthalpy becomes more exothermic as the attraction becomes stronger.
Factors affecting Enthalpy Change of Hydration
Ionic radius – as the ionic radius increases, the enthalpy change of hydration becomes less exothermic as the attraction on water molecules becomes weaker.
Ionic charge – as the ionic charge increases, the enthalpy change of hydration becomes more exothermic as the attraction on water molecules becomes stronger.
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